This invention relates to switching circuits and particularly to "power-on reset" circuits.
A power on reset circuit, as that term is used herein, is a circuit which produces a momentary output signal upon initial application of power to the circuit or upon interruption of the power. Such circuits may be used, for example, for placing counters, registers, memories or other circuits in a desired initial condition.
Selection of a power on reset circuit for a particular application may involve one or more of the following criteria: static and dynamic response, operating voltage range, steady-state power dissipation, and ease of implementation in integrated circuit form. The static and dynamic response should be such that the circuit operation is essentially independent of the supply voltage rise time. The operating voltage range should be such that the reset circuit does not require more voltage than the device with which it is to be used which otherwise would limit the operating range of the utilization device. The steady state power dissipation should be minimal, and ideally, zero. Finally, the circuit should not require physically large elements (such as high valued capacitors) which would make implementation as an integrated circuit difficult if not impossible.
Known circuits which meet many of the above criteria are shown and described in U.S. Pat. No. 3,809,926 titled "Window Detector Circuit" which issued to A. W. Young on May 7, 1974, in U.S. Pat. No. 4,045,688 titled "Power-on Reset Circuit" which issued to R. G. Stewart in Aug., 1977 and in U.S. Pat. No. 4,594,518 titled "Voltage Line Sensing Circuit" which issued to R. G. Pollachek on Jun. 10, 1986.
The present invention is directed to a new and improved circuit which includes means for sensing when the operating voltage increases above a first threshold value and means for sensing when the operating voltage decreases below a second threshold value. The circuit of the invention also includes additional advantageous operating characteristics discussed in detail below.